The T2x wifi-lid is different from a non-wifi lid.
The T2x wifi-lid has "antenna windows" (for lack of a better word) left and right.
Theoretically many antennae from different laptops could fit, but they wouldn't really work properly.
The non-wifi lid's material doesn't let enough wifi-signal through to be effective.

And if I could a related question – are antennas fairly easy to come by?

I've looked on ebay (UK) and while I can see antennas for other ThinkPad models, there's nothing for the T23. I wonder if I antennas from any other models would be compatible with the T23?

Cheers.

You can adapt pretty much any of the antennas on ebay into the T23. Fit one under the keyboard and the other under the hinge cover or on top of the LED using the aperture where the webcam connector is.
Alternatively I have a working T23 with factory fit wifi T23 and a TP-Link mPCI adapter available for a few quid if you are interested.

I was reading online and found some articles that suggest USB 1.1 severely limits the transfer speed of wireless internet.

I have a DWL-G630 that I bought new about 10 years ago, and it has the Atheros AR5005G chipset. The original version 3.00/3.05 drivers are severely out of date and do not support WPA2: http://sdfox7.com/dwlg630/

However, I found updated drivers online by reading info on this site. The drivers are dated 9/30/2009 and are version 7.7.0.406.

I downloaded the drivers it linked to (atheros_ar5xxx_770406_xp.zip) and this card now supports and connects to WPA2 without a problem. Upon testing, it certainly seems like the PCMCIA is performing at faster speeds than Wireless N on USB 1.1.

You can adapt pretty much any of the antennas on ebay into the T23. Fit one under the keyboard and the other under the hinge cover or on top of the LED using the aperture where the webcam connector is.

Thanks to those who answered that. I think I'll pick one up on eBay and get tinkering...

Alternatively I have a working T23 with factory fit wifi T23 and a TP-Link mPCI adapter available for a few quid if you are interested.

It's very kind of you to offer, but my collection of old ThinkPads has grown at an alarming rate recently (I bought a few spares-or-repairs T60p's from which to build a T601f), so I'd better not go for that just now!

To be honest, I think a N card is unnecessary for a T23, as the CPU then becomes the bottleneck even if you opt for the mPCI variant.
My A30p on Ethernet returns a speed of 16mb/s before the CPU reaches 100% and cap it at that speed.
EDIT: on best, best case scenario, I got 46mb/s on Speedtest with CPU load all the way up to 100% and soon afterwards the browser crashes.

I got inconsistent results on Speedtest using my school's wifi. The page also seems to be poorly written/coded. The multiple advertisements probably don't help; I'm not sure if they contaminate the download speed if they are downloading during the test.

The beta page is worse than the legacy Flash version, the upload test never happened. Tested on Firefox 45.9.1 ESR with Firefox 52 user agent.

I got inconsistent results on Speedtest using my school's wifi. The page also seems to be poorly written/coded. The multiple advertisements probably don't help; I'm not sure if they contaminate the download speed if they are downloading during the test.

Personally, I like K-Meleon. It is ultra-light weight and you can easily switch off ads, javascript, cookies, popups and images on the fly.

Thinkpad4by3's Law of the Universe.

The efficiency of two screens equally sized with equal numbers if pixels are equal. The time spent by a 4:3 user complaining about 16:9 is proportional to the inefficiency working with a 16:9 display, therefore the amount of useful work extracted is equal.

Just following up, it's been about a month. According to the battery applet on the Windows taskbar, I am averaging about 3 hours on the battery with the default IBM power settings. This is going to fluctuate between 2.5 and 3 if I have a browser window and Office document open, which is my typical usage.

I'm sure the fact that the Speedstep can clock down to ~500MHz during "idle" doesn't hurt!

I got inconsistent results on Speedtest using my school's wifi. The page also seems to be poorly written/coded. The multiple advertisements probably don't help; I'm not sure if they contaminate the download speed if they are downloading during the test.

The beta page is worse than the legacy Flash version, the upload test never happened. Tested on Firefox 45.9.1 ESR with Firefox 52 user agent.

Just following up, it's been about a month. According to the battery applet on the Windows taskbar, I am averaging about 3 hours on the battery with the default IBM power settings. This is going to fluctuate between 2.5 and 3 if I have a browser window and Office document open, which is my typical usage.

I'm sure the fact that the Speedstep can clock down to ~500MHz during "idle" doesn't hurt!

I was reading online and found some articles that suggest USB 1.1 severely limits the transfer speed of wireless internet.

I have a DWL-G630 that I bought new about 10 years ago, and it has the Atheros AR5005G chipset. The original version 3.00/3.05 drivers are severely out of date and do not support WPA2: http://sdfox7.com/dwlg630/

However, I found updated drivers online by reading info on this site. The drivers are dated 9/30/2009 and are version 7.7.0.406.

I downloaded the drivers it linked to (atheros_ar5xxx_770406_xp.zip) and this card now supports and connects to WPA2 without a problem. Upon testing, it certainly seems like the PCMCIA is performing at faster speeds than Wireless N on USB 1.1.

Hi everyone, I actually found newer drivers, 7.7.0.523, which also work with Windows 2000. (I tried the newest 9.2.0.104 but those are only for Windows XP.) You must use 7.7.406 or 7.7.523 to have WPA2 support under Windows 2000!